Old Soldiers is an older book but one I just got around to reading. It is another foray by David Weber into the Concordiat universe created by Keith Laumer and populated by the sentient AI tanks known as BOLOs.
I you have read Weber’s earlier book Bolo! then you will understand the back story of the two main characters. Menaka Trevor and the BOLO Lazarus. Both were featured in a novella in that anthology. This book picks up after the events in BOLO! with what the Concordiat does with Trevor and Lazarus after they are the only survivors f their battalion following the defense of the planet Chartres against a Melconian attack.